US8674307B2 - System and method for detecting infrared radiation - Google Patents

System and method for detecting infrared radiation Download PDF

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US8674307B2
US8674307B2 US13/277,661 US201113277661A US8674307B2 US 8674307 B2 US8674307 B2 US 8674307B2 US 201113277661 A US201113277661 A US 201113277661A US 8674307 B2 US8674307 B2 US 8674307B2
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bolometer
bolometers
current
circuitry
voltage
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Benoît Dupont
Michel Vilain
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Ulis SAS
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J5/00—Radiation pyrometry, e.g. infrared or optical thermometry
    • G01J5/10—Radiation pyrometry, e.g. infrared or optical thermometry using electric radiation detectors
    • G01J5/20—Radiation pyrometry, e.g. infrared or optical thermometry using electric radiation detectors using resistors, thermistors or semiconductors sensitive to radiation, e.g. photoconductive devices
    • G01J5/22—Electrical features thereof
    • G01J5/24—Use of specially adapted circuits, e.g. bridge circuits
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J5/00—Radiation pyrometry, e.g. infrared or optical thermometry
    • G01J5/10—Radiation pyrometry, e.g. infrared or optical thermometry using electric radiation detectors
    • G01J5/20—Radiation pyrometry, e.g. infrared or optical thermometry using electric radiation detectors using resistors, thermistors or semiconductors sensitive to radiation, e.g. photoconductive devices
    • G01J5/22—Electrical features thereof
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N23/00—Cameras or camera modules comprising electronic image sensors; Control thereof
    • H04N23/20—Cameras or camera modules comprising electronic image sensors; Control thereof for generating image signals from infrared radiation only
    • H04N23/23—Cameras or camera modules comprising electronic image sensors; Control thereof for generating image signals from infrared radiation only from thermal infrared radiation
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N25/00—Circuitry of solid-state image sensors [SSIS]; Control thereof
    • H04N25/60—Noise processing, e.g. detecting, correcting, reducing or removing noise
    • H04N25/67—Noise processing, e.g. detecting, correcting, reducing or removing noise applied to fixed-pattern noise, e.g. non-uniformity of response
    • H04N25/671—Noise processing, e.g. detecting, correcting, reducing or removing noise applied to fixed-pattern noise, e.g. non-uniformity of response for non-uniformity detection or correction
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N25/00—Circuitry of solid-state image sensors [SSIS]; Control thereof
    • H04N25/60—Noise processing, e.g. detecting, correcting, reducing or removing noise
    • H04N25/67—Noise processing, e.g. detecting, correcting, reducing or removing noise applied to fixed-pattern noise, e.g. non-uniformity of response
    • H04N25/671—Noise processing, e.g. detecting, correcting, reducing or removing noise applied to fixed-pattern noise, e.g. non-uniformity of response for non-uniformity detection or correction
    • H04N25/673—Noise processing, e.g. detecting, correcting, reducing or removing noise applied to fixed-pattern noise, e.g. non-uniformity of response for non-uniformity detection or correction by using reference sources
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N5/00—Details of television systems
    • H04N5/30—Transforming light or analogous information into electric information
    • H04N5/33—Transforming infrared radiation

Definitions

  • the present invention relates to the field of infrared imaging and pyrometry using bolometers. More especially, the invention relates to the field of image sensors for bolometric detection, regardless of the detection band and the type of bolometric materials used.
  • Detectors designed for infrared imaging are conventionally produced as a one or two-dimensional array of elementary detectors, or bolometers, said bolometers taking the form of membranes suspended above a substrate which is generally made of silicon, by means of support arms that have a high thermal resistance.
  • the substrate usually incorporates means of sequentially addressing the elementary detectors and means of electrically exciting and pre-processing the electrical signals generated by these bolometers.
  • This substrate and the integrated means are commonly referred to as the “readout circuit”.
  • the scene is projected through suitable optics onto the array of bolometers and clocked electrical stimuli are applied via the readout circuit to each of the bolometers or to each row of such bolometers in order to obtain an electrical signal that constitutes an image of the temperature reached by each of said elementary detectors.
  • This signal is then processed to a greater or lesser extent by the readout circuit and then, if applicable, by an electronic device outside the package in order to generate a thermal image of the observed scene.
  • This type of detector has numerous advantages in terms of its manufacturing cost and implementation but also has drawbacks that limit the performance of systems that use such detectors.
  • problems with regard to the uniformity of the image obtained In fact, when exposed to a uniform scene, not all the bolometers respond in exactly the same way and this results in fixed spatial noise in the image thus obtained.
  • This variability has several sources.
  • technological variability of the resistance of the bolometers causes, among other defects, offset variation and gain variation in the image, i.e. in the case of offset, spatial variation in the output levels of bolometers exposed to a uniform image and, in the case of gain, variability of the absolute variation in the output levels of bolometers that are exposed to a uniform temperature variation of a scene.
  • a first method for correcting offset variation involves using offset correction tables which are prepared after factory calibration operations.
  • the stability of these corrections depends on the temperature stability of the focal plane and thus, in non-temperature controlled applications (commonly referred to as “TEC-less”), it is necessary to resort to acquiring and storing gain and offset tables for multiple, so-called calibration temperatures and then using said tables when the detector is operated, for instance by interpolation, in order to ensure continuous digital correction over the entire operational dynamic range, in terms of temperature, of the focal plane of the detector.
  • TEC-less non-temperature controlled applications
  • Another method involves acquiring an image of a uniform reference scene by closing a mechanical shutter. Once this image has been acquired, the shutter is opened and the reference image is stored and then digitally or analogically subtracted from the current images.
  • This method is more widely known as “shutter correction” or “one-point correction”. It has the advantage of enabling highly efficient correction at around the ambient temperature of the detector which was used to acquire the reference image and requires little memory and few computing resources.
  • this method involves using a mechanical shutter—a mechanical device which has a non-negligible cost, is relatively fragile because of the moving parts it contains and consumes energy. What is more, if operating conditions change and, more especially the thermal environment of the detector changes, the images acquired from the scene deteriorate due to the reappearance of offset variation and it is then necessary to acquire a reference image again by closing the mechanical shutter. In fact, the detector is unusable, at least for the time it takes to acquire the reference image.
  • Another offset correction method which is disclosed, for example, in document WO 98/47102, involves digitally processing a series of consecutive images contained in a rolling time window that includes enough frames to make it possible to extract a continuous component from the time window.
  • the spatial distribution of this continuous component which is similar to the offset distribution, is then digitally subtracted from the current acquired images.
  • offset correction methods are only applied once an image has been acquired and therefore correct the effects of offset variation in the image. Nevertheless, although offset variation impacts image quality as such due to the presence of noise that is independent of the scene, it also has effects on the dynamic range of the observable scene that these types of techniques do not correct.
  • FIG. 1 shows a basic detection and readout layout of the kind that is conventionally used in bolometric array detectors.
  • This basic layout comprises:
  • Bolometer 12 is subjected to infrared radiation IR originating from a scene and is connected to ground by a first terminal A.
  • Integrating circuit 18 comprises:
  • Picture element 10 also comprises a read switch 16 that can be controlled by means of a “Select” signal and is connected to the inverting input ( ⁇ ) of the operational amplifier and a first MOS injection transistor 14 , the gate of which is controlled by a voltage VFID so as to impose a voltage Vac across the terminals of bolometer 12 , the source of which is connected to a second terminal B of bolometer 12 and the drain of which is connected to the other terminal of read switch 16 .
  • a read switch 16 that can be controlled by means of a “Select” signal and is connected to the inverting input ( ⁇ ) of the operational amplifier and a first MOS injection transistor 14 , the gate of which is controlled by a voltage VFID so as to impose a voltage Vac across the terminals of bolometer 12 , the source of which is connected to a second terminal B of bolometer 12 and the drain of which is connected to the other terminal of read switch 16 .
  • Compensation circuit 20 used to compensate the common-mode current that flows through imaging bolometer 12 comprises a resistive compensation bolometer 28 made of the same material as imaging bolometer 12 .
  • Compensation bolometer 28 is essentially insensitive to radiation originating from the scene, for instance because it has a low thermal resistance relative to the substrate and is, optionally or alternatively, provided with an opaque shield 30 .
  • One of the terminals of compensation bolometer 28 is connected to a predetermined voltage VSK and its other terminal is connected to the source of a second MOS injection transistor 32 of circuit 20 .
  • the drain of injection transistor 32 is connected to the inverting input ( ⁇ ) of operational amplifier 22 and its gate is connected to a predetermined voltage GSK.
  • imaging and compensation bolometers 12 , 28 are biased by the control voltage of biasing transistors 14 , 32 and the difference between current lac that flows through imaging bolometers 12 and current Iav that flows through compensation bolometer 28 is integrated by integrating circuit 18 over a predetermined integration duration Tint.
  • the use of compensation circuit 20 is justified by the fact that the useful current, i.e. that which is representative of the temperature of the scene, only accounts for a minute portion, generally around 1%, of the total current that flows through imaging bolometer 12 , hence the need to eliminate the common-mode current before integration.
  • Vout VBUS + 1 C int ⁇ ⁇ 0 T ⁇ ⁇ int ⁇ ( Iac ⁇ ( t ) - Iav ⁇ ( t ) ) ⁇ ⁇ d t ( 1 )
  • circuit 18 thus makes it possible to apply, through the value of capacitance C int , gain to readout of the wanted signal whilst ensuring conversion of the useful current to a voltage that is simpler to manipulate. This way, all the imaging bolometers of the array detector are read in the same way, especially by applying the same bias level.
  • the spatial variation in voltages Vout after all the bolometers have been read is approximately 10%. In conventional detectors, this variation represents around 300 mV of their dynamic output response. If biasing of the imaging bolometers is also increased, for instance by 50%, in order to increase the value of the output levels and hence the sensitivity of the detector, the variation in output voltages Vout also increases by 50% and then reaches 450 mV. Considering that the total dynamic response available is usually limited to 2 or 3 V, a significant portion of this dynamic response is therefore used up by the natural variability of bolometers alone.
  • offset variation simply by existing, uses up a portion of the dynamic output response of a detector.
  • the term “residual dynamic response” or “dynamic scene response” is usually used to denote the difference between the maximum amplitude of voltage Vout when the integrating circuits are not saturated and the maximum amplitude of output voltages Vout when exposed to a uniform scene, i.e. the remaining dynamic response to the wanted signal.
  • the object of the present invention is to propose a method and a bolometric detection device that correct the effect of offset variability both on formed images and on residual dynamic response.
  • the object of the invention is a device for detecting infrared radiation comprising:
  • the system comprises circuitry for correcting the resistance of the bolometers which is capable of injecting current into each bolometer in order to shift its resistance by a predetermined quantity that depends on its offset, current injection being performed prior to readout biasing of the bolometer and the shift being performed according to the direction in which the bolometer's resistance varies as a function of temperature.
  • the circuit that controls the resistance of the bolometers according to the invention individually modifies the value of the resistances of the bolometers so as to reduce this resistance if the resistance of the bolometer diminishes as a function of temperature or increase it if the opposite applies.
  • the resistances of the bolometers are preferably modified in a way that makes them substantially identical. This correction is performed upstream from reading, i.e. before the bolometers are biased and the currents are integrated. This modification of resistances is obtained through the Joule effect by using electrical means in a phase prior to the integration phase without altering the thermal information obtained from the observed scene.
  • the device comprises one or more of the following aspects.
  • the correction circuitry is capable of shifting the resistances of the bolometers towards a common value.
  • the correction circuitry comprises timing means capable of stopping current injection after a duration that is determined as a function of the offset dependent quantity.
  • the bolometer comprises a bolometric membrane of the semiconductor type suspended above a substrate and timing means (142) is capable of stopping current injection after a period according to the equation:
  • t ⁇ ( i , j ) k ⁇ TPF 2 ⁇ C th E A ⁇ V a ⁇ ⁇ c 2 ⁇ ⁇ ⁇ ⁇ R 0 ⁇ ( i , j )
  • t(i, j) is the predetermined duration
  • ⁇ R 0 (i, j) is a quantity that depends on the offset
  • k is Boltzmann's constant
  • TPF is the temperature of the substrate
  • C th is the heat capacity of the bolometer
  • E A is the thermal conduction activation energy of the bolometric material of which the bolometer is made
  • V ac is the voltage across the terminals of the bolometer.
  • the correction circuitry comprises the following in order to inject current into the bolometer:
  • the comparison circuit comprises:
  • V ref ( i,j ) V clamp ⁇ I ref ⁇ R 0 ( i,j )
  • V ref (i,j) is the first voltage
  • V clamp is the second voltage
  • Iref is the constant current output by the constant current source
  • ⁇ R 0 (i, j) is the predetermined quantity that depends on the bolometer's offset.
  • the correction circuitry is capable of injecting a current into the bolometer having a value that depends on the predetermined quantity which depends on the bolometer's offset.
  • Iref ⁇ ( i , j ) k ⁇ TPF 2 E A ⁇ R a ⁇ ⁇ c ⁇ ( i , j ) ⁇ ⁇ ⁇ ⁇ t ⁇ C th ⁇ ⁇ ⁇ ⁇ R 0 ⁇ ( i , j )
  • Iref(i,j) is the value of the current
  • ⁇ R 0 (i, j) is the value that depends on the offset
  • k is Boltzmann's constant
  • TPF is the temperature of the substrate
  • C th is the heat capacity of the bolometer
  • E A is the thermal conduction activation energy of the bolometric material of which the bolometer is made
  • R ac (i,j) is the resistance of the bolometer
  • ⁇ t is the time for which the current is applied.
  • the circuitry that controls the resistance of the bolometers is capable of temporarily deferring the injection of current into the bolometers of a predetermined set of bolometers so as to terminate said current injection substantially simultaneously.
  • the array of bolometers is read one row at a time and the resistance control circuitry is located at the end of each column of the array of bolometers and is capable of being connected to every bolometer in the column in order to control each bolometer's resistance.
  • the invention thus makes it possible to increase the sensitivity of the detector by increasing its residual dynamic response and therefore reducing the extent of variability of the offsets in the formed images and/or to use the device at higher temperatures. More generally, the product of sensitivity times residual dynamic response is substantially increased.
  • the object of the invention is also a method for detecting infrared radiation by using an array of bolometers, this method consisting of the following steps in order to read a bolometer:
  • the method involves, prior to reading the bolometer, injecting a current into the bolometer so as to shift the bolometer's resistance by a predetermined quantity that depends on the latter's offset according to the direction in which the resistance of the bolometer varies as a function of temperature.
  • the predetermined values that depend on the offsets associated with the bolometers are determined by:
  • FIG. 1 is a schematic view of a basic detection and readout layout of the kind that is conventionally used in bolometric array detectors. This layout has already been described above;
  • FIG. 2 is a schematic view of a first embodiment of a bolometric array detection device in accordance with the invention
  • FIG. 3 is a flowchart showing a method for correcting variability in the offset of bolometers in accordance with the invention
  • FIG. 4 is a diagram that explains changes in the resistances as a function of time subsequent to the invention being applied;
  • FIGS. 5A and 5B are timing diagrams for various signals used to control switches and that are produced when the method according to the invention is used;
  • FIG. 6 is a schematic view of a first embodiment of a source of individual reference voltage Vref which forms part of the bolometric detection device according to the invention
  • FIG. 7 is a schematic view of a second embodiment of a source of individual reference voltage Vref in accordance with the invention.
  • FIG. 8 is a schematic view of a second embodiment of a bolometric array detection device in accordance with the invention which uses a source of individual reference current Iref;
  • FIGS. 9A and 9B are schematic views of devices for time-based control that are part of a third embodiment of a bolometric detection device in accordance with the invention.
  • a bolometric detector according to the invention is shown schematically in FIG. 2 .
  • This detector comprises an array 40 of unitary detection elements 42 , or “pixels”, having N rows and M columns with each of the pixels comprising an imaging bolometer 12 , a MOS transistor 14 and a read switch 16 .
  • Each column of array 40 is associated, via a column read bus 44 , with readout circuitry 46 which comprises an integrator 18 , formed by an operational amplifier 22 , capacitor 24 and zero reset switch 26 , as well as a compensation circuit 20 , formed by a resistive compensation bolometer 28 which is substantially insensitive to the radiation, for example by heat sinking to the substrate and/or by means of an opaque shield 30 , and a MOS injection transistor 32 .
  • readout circuitry 46 which comprises an integrator 18 , formed by an operational amplifier 22 , capacitor 24 and zero reset switch 26 , as well as a compensation circuit 20 , formed by a resistive compensation bolometer 28 which is substantially insensitive to the radiation, for example by heat sinking to the substrate and/or by means of an opaque shield 30 , and a MOS injection transistor 32 .
  • Each pixel 42 of array 40 forms, together with its associated readout circuitry 46 , a basic layout similar to that described in relation to FIG. 1 .
  • All the bolometric elements 12 , 28 are formed on the surface of substrate in which all the electronic elements are formed.
  • the optically active area 40 is placed at the focus of appropriate optics (not shown).
  • bolometers 12 of array 40 are read row by row, with the row of pixels that is currently being read being connected to readout circuitries 46 by closing read switches 16 .
  • signals Vout are sampled and held before addressing the next row and then multiplexed to output amplifier 49 ; readout is usually clocked by a timer circuit 48 that is provided in substrate and tasked with opening and closing read switches 16 and zero reset switches 26 .
  • the reader should refer, for instance, to the article mentioned above for more details of how readout operates.
  • each column of array 40 is also associated, via a column correction bus 50 , with circuitry 52 for controlling the resistance of the bolometers in said column.
  • the function of circuitry 52 is to correct the effect of the offset variability of bolometers 12 on the formed images as well as the effect of this variability on the detector's residual dynamic response. Connecting and disconnecting a pixel 42 in a column to its control circuitry 52 is ensured by a correction switch 53 located in pixel 42 between correction bus 50 and bolometer 12 and driven by timer circuitry 48 in a manner that is described in detail below.
  • Control circuitry 52 comprises:
  • the voltage produced by voltage source 66 is programmable in order to allow adaptation to suit the pixel to which control circuitry 52 is connected.
  • This voltage source comprises, for instance, a digital-to-analogue converter fed by a table of digital values stored in the detector.
  • the detector according to the invention comprises a correction management unit 69 that is typically (but not necessarily) not located on substrate 10 .
  • Management unit 69 stores, in particular, correction parameters for the resistances of imaging bolometers 12 and implements calibration of said parameters, as explained in greater detail below.
  • Unit 69 is, for instance, a digital processing unit as classically provided in detectors according to the prior art.
  • the detectors are actually provided, firstly, with a digital output (analogue-to-digital conversion (ADC)) for signals Vout formed in substrate 8 behind amplifier 49 or remoted in external electronic components and are, secondly, associated with a digital processing unit which comprises memories and correction algorithms, for example offset and gain algorithms for traditional “2-point corrections” that are needed for ordinary use of the detector.
  • ADC analog-to-digital conversion
  • Said digital processing unit is deemed, hereinafter, to comprise, as is customary in this field, said means of storage and means of processing the digital data stated above in order to implement the invention.
  • a method for correcting the effects of offsets as used by the detector described above is described below in relation to the flowchart shown in FIG. 3 .
  • This method is based on the rapid variation that the resistances of bolometers exhibit when a current flows through them (self-heating phenomenon due to the Joule effect). A current is thus injected into imaging bolometers 12 so as to individually correct their resistance before, and as close as possible to, the integration phase.
  • bolometers 12 taking the form of membranes suspended above a substrate for example, are of the semiconductor type with their material that is sensitive to temperature variations being amorphous silicon (a-Si) or a vanadium oxide that is generically designated “VOx”.
  • a-Si amorphous silicon
  • VOx vanadium oxide
  • the method according to the invention starts with a calibration phase 70 that is performed, for instance, at the factory and/or on a regular basis in order to take into account detector drift over the course of time.
  • This calibration phase 70 involves a first step 72 to expose the detector to a uniform scene for a given, constant focal plane temperature TPF.
  • TPF is the temperature to which the detector is adjusted when in use. It should also be noted that, since the substrate in which readout circuitry 46 and control circuitry 52 are formed and above which the bolometric membranes are formed is arranged in the focal plane of optics, this temperature will be referred to either as the temperature “of the focal plane” or the temperature “of the substrate”.
  • Array 40 of bolometers 12 is then read, row by row, in step 74 , with each row being successively connected to readout circuitries 46 located at the end of a column by closing read switches 16 which causes biasing of imaging bolometers 12 .
  • Connecting a row to circuitry 46 is preceded by discharging capacitors 24 by closing zero reset switches 26 and then opening them.
  • transistors 32 of compensation circuitries 20 are forced to the off state so as to cancel out common-mode currents and imaging bolometers 12 are biased with a low voltage in order not to saturate capacitors 24 of integrators 18 .
  • Voltages Vout on the output of integrators 18 that result from integrating the currents that flow through imaging bolometers 12 are then analyzed by management unit 69 in order to determine the corresponding resistances of bolometers 12 of array 40 in a manner that is known in itself from the prior art.
  • R a ⁇ ⁇ c ( R a ⁇ ⁇ c ⁇ ( 1 , 1 ) R a ⁇ ⁇ c ⁇ ( 1 , 2 ) ... R a ⁇ ⁇ c ⁇ ( 1 , M ) R a ⁇ ⁇ c ⁇ ( 2 , 1 ) R a ⁇ ⁇ c ⁇ ( 2 , 2 ) ... R a ⁇ ⁇ c ⁇ ( 2 , M ) ⁇ ⁇ ⁇ ⁇ R a ⁇ ⁇ c ⁇ ( N , 1 ) R a ⁇ ⁇ c ⁇ ( N , 2 ) ... R a ⁇ ⁇ c ⁇ ( N , M ) ) ( 2 )
  • This table is then stored in management unit 69 .
  • Calibration phase 70 then continues by determining, by means of management unit 69 , a resistance correction quantity for each of imaging bolometers 12 in step 76 .
  • Resistance R min is the target resistance of the correction according to the invention to which the resistances of imaging bolometers 12 are adjusted before readout biasing in the special case where the detector is once more placed in the same uniform thermal illumination conditions as in calibration phase 70 .
  • the same individual resistance corrections will be applied using the same method. This results in elimination of resistances that are independent of the scene, as is also described in detail below.
  • step 76 a table ⁇ R 0 of individual correction quantities ⁇ R 0 (i, j) for the resistances of bolometers 12 in accordance with the following tabulated equation:
  • Calibration phase 70 then completes by management unit 69 storing array ⁇ R 0 .
  • FIG. 4 illustrates changes in the resistances of a single row over time when the invention is implemented. For the sake of clarity, these changes correspond to exposing the detector to a uniform scene and it is assumed that the distribution of the resistances only includes contributions of a technological nature and the thermal distribution of the substrate, i.e. those equivalent to the conditions in calibration phase 70 . Obviously, for ordinary use when exposed to an arbitrary scene which is the point of interest of the invention, what happens to the population of resistances will be commented on.
  • FIG. 5A shows the control signals of the various switches that are produced when a first embodiment of the method according to the invention is used by the system in FIG. 2 .
  • a correction phase 80 for the resistances of the bolometers 12 in a row of array 40 is performed before and as close as possible in time to the readout phase 82 of said row.
  • the correction phase 80 of a row of array 40 starts in 84 by adjusting each of the voltages Vref of control circuitries 52 .
  • Voltage Vref of a circuitry 52 is then adjusted to an individual value relative to pixel 12 of the column associated with circuitry 52 as explained in more detail below.
  • correction phase 80 continues, in 86 , by closing the correction switches 53 of the row of pixels with the selection switches 16 of the latter remaining opened.
  • switches 56 that are connected to current sources 54 are then closed by a brief “Start” pulse provided by sequencer 48 on the “Check” command, through an OR gate, the role of which will be clarified further below, so that a current having the value Iref flows through bolometers 12 .
  • the closing of switches 56 marks the timeline “0” origin point.
  • R bolo R ac .
  • switches 64 which are connected to capacitors 60 are also closed by the “Start” pulse and the effect of this is to bring, almost immediately, the (+) input of comparator 62 and the armature of capacitor 60 which is connected to it to potential Vclamp.
  • step 88 is sufficiently short, relative to the total duration of correction phase 80 , to allow one to consider that, during step 88 , resistance R bolo of bolometers 12 of the row that is currently being corrected changes little despite the Joule effect caused by biasing them.
  • the duration of step 88 essentially depends on the value of the capacitance of capacitors 60 and the value of current Iref and is, by way of example, around 500 nanoseconds.
  • step 90 the state of the switches 64 that are connected to capacitors 60 is changed to the open state when the “Start” pulse returns to its low level. Note that, at this stage, capacitors 60 do not discharge and keep the voltage difference Vcap constant across their terminals and these voltage differences will be preserved until the “Start” signal is subsequently activated because the branch connected to the positive input (+) of comparator 62 of circuitry 52 has a very high impedance.
  • FIG. 4 schematically shows this reduction as a linear, first-order approximation. Also note that if injected current Iref is comparable to the bias current used during integration, the slopes dR/dt of the two segments are comparable during the correction and integration phases, as shown in FIG. 4 .
  • Voltage V + therefore follows the variation ⁇ R bolo (t) in the resistance of bolometer 12 due to the effect of its self-heating and therefore diminishes as a function of time.
  • voltage V + on the positive input (+) of comparator 62 exceeds voltage Vref on its negative input ( ⁇ ), materialised by step 92 , the “check” signal that controls switch 56 which is connected to source 54 is held in a high state through OR gate 63 . Switch 56 therefore remains closed and current Iref continues to flow through corresponding bolometer 12 and voltage V + continues to diminish.
  • ⁇ R 0 (i,j) corresponds to the individual value of array ⁇ R 0 relative to bolometer 12 of the pixel 42 to which circuitry 52 is connected.
  • FIG. 4 shows, in particular, changes over time in the highest resistance (R bolo MAX), the lowest resistance (R bolo MIN) and any intermediate resistance (R bolo (i,j)) of a single row i that is in the process of being corrected.
  • Current injection stops (the resistance no longer diminishes) in each resistance at instants that are defined by the respective values ⁇ R 0 (i, j) in accordance with the stated principle.
  • the “Correction” signal controlled, preferably in an adjustable manner, by sequencer 48 is held in a high state for a duration that is sufficient for the highest resistance (equals R bolo MAX at the zero instant) to have time to vary by the highest quantity ⁇ R 0 in table ⁇ R 0 (i,j).
  • all the resistances finish changing in step 94 at value R min .
  • the method then continues with readout phase 82 during which zero reset switches 24 of readout circuitries 46 are closed and then reopened in 98 (not shown in FIG. 5A because this operation runs in parallel with step 80 ) in order to discharge capacitors 24 of integrators 18 , then the read switches 16 of pixels 12 of the row that is currently being read are closed in step 100 by activating the “Select” command in order to connect the pixels 12 of the row to circuitries 46 for a duration that defines the predetermined integration time Tint as explained above.
  • step 94 and step 100 which is variable depending on the bolometer in question, the latter's temperature tends to return to its equilibrium value at a rate that is limited by the thermal time constant of the bolometers; this results in slight restoration of the natural variations in resistance, typically accompanied by inversion of the distribution order, as indicated in FIG. 4 , without this having any insurmountable adverse effect, given the very small time periods in question (several microseconds).
  • Step 82 of the method then loops to step 84 in order to correct the resistances and read the bolometers of the next row i+1 of array 40 .
  • a sample and hold operation (not shown in FIG. 5A ) is performed on voltages V out of row i.
  • Multiplexing of the stream of signals to output amplifier 49 may, if necessary, extend into the integration phase for row i+1, as is known in itself.
  • FIG. 4 shows the attraction of setting value R min below the set of values R ac (i,j). Adopting this procedure, the values of all the resistances are modified before the start of the integration step by injecting current Iref. If value R min is set too high, some of the bolometers will actually not be affected by the resistance adjustment and this would produce image distortion due to a local correction defect. On the other hand, if this value is set significantly below the minimum distribution value, this will necessitate a pointlessly long duration in order to obtain standardisation of all the resistances and this would have nothing but disadvantages.
  • the time periods that are not specified in FIG. 4 are defined on the basis of nominal signal settling criteria, as is customary in the profession, without any other particular constraints.
  • a resistance correction typically takes 4 to 8 microseconds in order to apply a resistance offset of 2% due to the Joule effect. More generally, the time during which current Iref is applied is several microseconds; this time is short compared with the usual integration times Tint which last several dozen microseconds or the duration of a read frame which usually equals 16 ms. Note also that it is possible to shorten the duration of correction by applying a higher current Iref without having to modify the circuits or operation described above.
  • FIG. 4 shows the population of resistances of imaging bolometers 12 of array 40 before they are corrected (histogram “A”) and after correction (histogram “B”) in a case where the thermal conditions are representative of the calibration phase. Note that the variability of the resistances of bolometers 12 is substantially reduced before they are read, by correction in accordance with the invention. Thus, the effect of offset variability on formed images is substantially lessened and the residual dynamic response is substantially increased.
  • this histogram “B” before integration step 100 is less than that of uncorrected histogram “A” (by the width of the amplitude of the corrections) and this results in appreciable gain in the dynamic scene response of the detector.
  • correction is therefore similar to a “1-point” type offset correction as described above, i.e. comparable to closing a mechanical shutter followed by associated corrections and at the continuous level of voltages Vout on the output of integrators 18 .
  • correction according to the invention does not impose any limitation in terms of dynamic response to the signal on the detector's output.
  • by reducing the variability of the resistances and thus the variability of bias currents Iac that flow through bolometers 12 when they are read it is possible to improve the residual dynamic response and/or increase the bias voltage of bolometers 12 , as the user chooses.
  • FIGS. 6 and 7 show examples of embodiments of programmable voltage source 66 .
  • Voltage source 66 comprises a capacitor 110 that is connected between the negative terminal of comparator 62 of control circuitry 52 and ground and a controllable switch 112 that is connected between said negative input and an analogue multiplexing bus 114 .
  • This bus 114 is itself connected to a digital-to-analogue converter 116 which receives, on its input, a digital value, e.g. in n bits, of voltage Vref that needs to be produced by voltage source 66 .
  • This digital voltage value is supplied by associated management unit 69 to the detector and sent over bus 114 in serial mode during step 84 , as detailed below.
  • Analogue multiplexing can be used to implement the invention despite temporal changes to pre-charged voltages Vref due to slight leakage currents because the useful time for which these voltages are maintained corresponds roughly to 1 row time, i.e. several dozen microseconds.
  • Voltage source 66 comprises a digital-to-analogue converter 122 whose output is connected to the negative input of comparator 62 of control circuitry 52 , an n-bit digital register whose output is connected to the input of converter 122 , and a controllable switch 126 which is connected between the input of register 124 and an n-bit digital multiplexing bus 128 .
  • a digital value for this voltage is initially produced on bus 128 by management unit 69 which is associated with the detector and then switch 126 is closed.
  • the digital value on bus 128 is then stored in register 124 and converted by converter 122 into voltage Vref.
  • switch 126 is opened, leaving bus 128 free to be used in order to adjust voltage Vref of another control circuitry 52 .
  • the time needed to obtain correction of the resistances of all the bolometers 12 of a row of array 42 depends on the correction quantities ⁇ R 0 (i,j). Since values ⁇ R 0 (i,j) are not equal, the correction time therefore varies from one bolometer to another. Also, the bolometers in a row are necessarily read synchronously. Consequently, the duration of correction phase 80 must be chosen so that all the corrections of the resistances of the bolometers in a single row are effective and that the read phase 82 of this row starts without waiting unduly after the end of correction phase 80 .
  • system described above can be modified in accordance with a second embodiment which differs from the embodiment described in relation to FIG. 2 in terms of the current sources of control circuitries 52 which output currents Iref depending on the corrections that are to be made in order to obtain an equal correction duration ⁇ T for all the bolometers.
  • a table of currents Iref(i,j) is calculated on the basis of the table of corrections ⁇ R 0 (i,j), for example in accordance with the equation proposed below, and module 52 is reduced to current generator 54 which is associated with means of programming current Iref and has the layout shown in FIG. 8 for instance.
  • the functional timing diagram shown in FIG. 5A boils down to the “Correction” signal which is changed to a high state on duration ⁇ T.
  • This embodiment does not require any switches 56 because the time ⁇ T for which current Iref is applied can be defined directly by the state of switch 53 which is controlled by sequencer 48 .
  • the correction phase 80 for a row i of array 50 starts in 84 by individually digitally programming the currents that are to be produced by each of the generators 54 of circuitries 52 .
  • Correction phase 80 continues in 86 which corresponds to closing the switches 53 of the row of pixels with the selection switches 16 of the latter remaining opened.
  • This phase finishes in 96 by opening switches 53 (the “correction” command returns to a low state) after a predetermined duration ⁇ t. The rest of the process is identical to that described earlier.
  • Loading the current values is typically realised digitally by using multiplexing similar to that described in relation to FIG. 7 and shown schematically in FIG. 8 .
  • the DAC controls one current generator per column with the value of the currents output by generators 54 of each column being updated one row at a time.
  • a third embodiment is described below. This embodiment differs from the embodiment in FIG. 2 by virtue of the control circuits for the resistances 52 of bolometers 12 , as illustrated in FIG. 9A .
  • a control circuit 52 comprises, like a control circuit 52 in the first embodiment, a current source 54 capable of outputting a current having a constant value Iref and connected to column bus 50 via a switch 56 that is controlled by an OR logic gate, one input of which is controlled by a “Start” command.
  • the other input follows the output of an interval timer 142 .
  • the value of duration t(i,j) is pre-programmed in n bits in timer 142 as a function of the pixel 12 to which control circuit 52 is connected. Timer countdown is triggered by the “Start” command on timer 142 which confirms the high state of the “Check” signal via the OR gate when the “Start” pulse changes back to the low state.
  • the timing diagram for the digital signals in FIG. 5A applies at every point.
  • the temperature difference ⁇ (i, j), corresponding to ⁇ R 0 (i, j), to be reached for the most resistive bolometer is consequently very small and is of the order of one degree. Such a temperature rise is achieved within several microseconds if the bolometer is subjected to a current of roughly the same order as that which is usually used to read the bolometer. This duration is negligible compared with the thermal time constant of the bolometer which is usually 5 ms to 15 ms.
  • the invention aims to impose a variation in resistance beyond a reference point where the equilibrium temperature when exposed to the incident infrared radiation is reached, the term Pir is therefore not involved in the duration of the bolometer's temperature rise.
  • t ⁇ ( i , j ) k ⁇ TPF 2 ⁇ C th E A ⁇ V a ⁇ ⁇ c 2 ⁇ ⁇ ⁇ ⁇ R 0 ⁇ ( i , j ) ( 13 )
  • table ⁇ R 0 of individual resistance correction quantities is replaced by table ⁇ T 0 which contains the temperature-rise durations t(i, j) that are to be applied in order to produce said quantities.
  • the correction phase for a row of array 40 of pixels 12 thus involves loading the corresponding duration values in interval timers 142 , closing switches 56 (switches 16 remain open and switch 53 is closed) and then opening each of them after an individually customised duration t(i, j).
  • the resistance of each of bolometers 12 of a row is thus corrected by their corresponding quantity ⁇ R 0 (i, j).
  • the first embodiment described in relation to FIG. 2 is preferred insofar as instants t(i, j), regardless whether they are calculated by a solver or in accordance with equation (13), depend on the value C th .
  • Using a single value of C th for all bolometers 12 is an approximation that may lead to measurement inaccuracy insofar as this parameter of bolometers exhibits natural technological variability.
  • TPF temperature of the focal plane
  • This third embodiment can advantageously be associated with operation whereby step 94 which corresponds to the end of current injection is simultaneous for all circuits 52 .
  • Such operation is obtained by closing switch 56 of each circuit 52 after a wait time that corresponds to the additional time t(i,j) relative to time t(i,j)MAX which corresponds to the bolometer that is initially the most resistive in table ⁇ T 0 .
  • FIG. 9B shows a version of control circuit 52 which produces this result, in relation to the timing diagram in FIG. 5B .
  • FIG. 5B only the “Check(R bolo MIN)”, “Check(R bolo MAX)” and “Check(R bolo (i,j))” signals are shown, the “Start”, “Correction” and “Select” signals being identical to those in FIG. 5A .
  • FIG. 9B the OR gate used in FIG. 9A is replaced by a NOR gate and the output polarity of timer 142 is reversed.
  • table ⁇ T 0 which is used to preload timers 142 is replaced by additional table C ⁇ T 0 in which each element equals [t(i,j)MAX ⁇ t(i,j)].
  • the “Start” signal initiates countdown of the individual times for each timer 142 which closes associated switch 56 when the additional time Ct(i,j) has elapsed.
  • Closing switch 53 (“Correction” signal in low state) marks the end of current injection and of correction step 80 for all the bolometers in a single row i at the same instant.
  • This embodiment will therefore typically only be preferred, to the extent that the circuit is simpler compared with that in FIG. 2 given as an example of realisation according to the first embodiment, if the technological variability of parameter Cth can be considered to produce negligible variability of output signals Vout with regard to the effect of other variations that are independent of the scene.
  • ⁇ ⁇ ⁇ R 0 ⁇ ( i , j ) E A k ⁇ TPF 2 ⁇ ⁇ ⁇ ⁇ t ⁇ R a ⁇ ⁇ c ⁇ ( i , j ) ⁇ Iref 2 ⁇ ( i , j ) C th ( 15 )
  • Iref ⁇ ( i , j ) k ⁇ TPF 2 E A ⁇ R a ⁇ ⁇ c ⁇ ( i , j ) ⁇ ⁇ ⁇ ⁇ t ⁇ C th ⁇ ⁇ ⁇ ⁇ R 0 ⁇ ( i , j ) ( 16 )
  • This expression allows numerical estimation of each current Iref(i,j) that is to be applied for the chosen duration ⁇ t.
  • This implementation of the invention has the advantage of simultaneously setting all the resistances of bolometers 12 to values, before integration, that are devoid of variations that are independent of the scene on the row that is currently being processed at an instant that is preferably very close to the start of the integration phase. There is then essentially not enough time for variations associated with thermal relaxation towards equilibrium to manifest themselves.
  • curves R bolo (t) in a diagram such as that in FIG. 4 show a series of segments having different slopes which converge at a single point having the value R min at the end of duration ⁇ t.
  • Duration ⁇ t is adjustable thanks to the associated setting of currents Iref(i,j) to a value that is, firstly, appropriate to the desired correction accuracy and, secondly, does not extend the row time, e.g. below the time required for the sample and hold phase that precedes and follows each row integration phase.
  • bolometers made of a bolometric material whose resistance diminishes as its temperature rises, i.e. a bolometric material that has a negative resistance coefficient, such as a semiconductor material consisting of amorphous silicon (a-Si) or vanadium oxide (VOx).
  • a-Si amorphous silicon
  • VOx vanadium oxide
  • the invention also applies to bolometric materials that have a positive resistance coefficient, i.e. materials whose resistance increases as their temperature rises, such as metals, especially titanium.
  • correction in accordance with invention involves increasing the resistance of each bolometer rather than reducing it as described above so as to obtain a distribution that is substantially concentrated around resistance R max of table R ac of resistances obtained during the calibration phase.
  • Embodiments of the invention that apply to temperature-controlled bolometric detectors are also described above.
  • TEC-less detectors that are not temperature controlled and more commonly referred to as “TEC-less”.
  • the correction quantities ⁇ R 0 (i, j) are adjusted at the start of the correction phase as a function of the temperature of the focal plane, measured by a sensor located on the substrate, e.g. a semiconductor sensor formed directly in the readout circuitry.
  • a sensor located on the substrate e.g. a semiconductor sensor formed directly in the readout circuitry.
  • a plurality of correction tables ⁇ R 0 having a respective duration ⁇ T 0 , are acquired as a function of several focal plane temperatures TPF and stored in the detector.
  • one particular table among these stored tables is selected or interpolated as a function of the measured temperature TPF mes of the focal plane. This design, however, makes it necessary to acquire several reference tables and this can be a long and therefore expensive process.
  • a single reference table Rac is acquired during the detector's calibration phase. Because there is a known model for variation of the resistance of bolometers as a function of temperature, e.g. equation (7), a table of resistances Rac TPFmes is calculated regularly and/or periodically when using the detector as a function of the measured temperature TPF mes of the focal plane and of reference table R ac . Using equation (7), the individual values Rac TPFmes (i,j) are then calculated in accordance with the equation:
  • correction table ⁇ R 0 is determined then the corresponding reference voltages Vref, or currents Iref(i,j), or corresponding temperature-rise durations t(i, j), as described above.
  • the spatial variation of the temperature of the focal plane can also be taken into account by using several temperature sensors located on the focal plane. Spatial modelling of the focal plane temperature is then used as a function of the temperature measurements in order to determine the temperature of the substrate at the level of each imaging bolometer. The quantity needed to correct the resistance of a bolometer is then calculated as a function of the corresponding temperature of the substrate.

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CN108132101A (zh) * 2017-12-15 2018-06-08 中国电子科技集团公司第四十四研究所 抗辐照高动态焦平面读出电路
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